1. Field of the Invention
This invention relates generally to the removal of molybdenum (Mo) metal from substrates by etching such as the removal of Mo metal debris from surfaces such as ceramic and metal feature surfaces of sintered ceramic substrates in the fabrication of ceramic electronic components containing molybdenum circuitry and in the fabrication of molybdenum photomasks. In particular, this invention is concerned with an environmentally friendly process for removal of extraneous molybdenum metal debris from substrates involving a Mo etching step and an oxide removal step to strip a black oxide surface layer formed on the metal features during the etching step.
2. Description of Related Art
Production of ceramic substrates such as alumina multilayer ceramics with interconnected metal vias for packaging semiconductor devices requires screening of an electrically conductive paste such as a metal-polymer composite paste through a mask, such as a metal mask, to deposit conductive patterns onto ceramic green sheets. After screening, the green sheets are dried, assembled, laminated, cut to size, and the laminates are sintered. A commonly used conductive paste used in alumina ceramic substrate technology is a molybdenum metal based paste which is a dispersion of molybdenum metal powder in an organic binder, such as, for example, ethyl cellulose, polymethyl methacrylate, and the like, a high boiling solvent vehicle system in conjunction with plasticizers, dispersants/surfactants, and other conventional additives. It is generally observed that the as-processed sintered ceramic substrates have the problem of varying levels of lodged-in extraneous Mo metal debris as particles or flakes on the surface of the ceramic dielectric and also on the sintered Mo metal conductive features. A pre-clean step prior to further processing, for example, deposition of joining metallurgy, is therefore a standard part of the overall process. FIG. 1A shows a ceramic substrate containing Mo metal filled vias and in which extraneous Mo is on the ceramic surface. It is the Mo debris on the sintered ceramic surface which must be removed to provide a substrate suitable for subsequent operations.
A commonly used pre-clean process involves vapor blast or bead blast treatment of sintered parts followed by contacting with a potassium ferricyanide (K3[Fe(CN)6])/KOH solution at high pH (about 13) to etch extraneous Mo from the substrate surface. Potassium ferricyanide at high pH is an efficient etchant for Mo metal which has no problem of oxidation of Mo or black via formation and is a standard etchant in the fabrication of Mo metal masks for screening conductive paste on substrates. However, the use of potassium ferricyanide in production scale processes, causes environmental issues with the etch bath solution handling and disposal of cyanide contaminated waste as cyanide carrying iron complexes. The waste issue can be partially addressed by using recycling technologies, such as etchant bath regeneration by oxidation with ozone, thus minimizing etch waste but there is still a disposal problem with large volumes of cyanide-contaminated rinse water.
U.S. Pat. 5,518,131 (Chen et al.) describes an alternate method for etching molybdenum in metal mask fabrication that utilizes ferric sulfate or ferric ammonium sulfate as etchants as a replacement of ferricyanide. These reagents provide Mo etch rates comparable to ferricyanide under optimized process conditions and at the same time have the advantage of having a non-hazardous easily treatable waste.
It has been found, however, that when using ferric sulfate or ferric nitrate or ferric ammonium sulfate aqueous solutions for removal of extraneous Mo from ceramic substrates, the etch removal of Mo debris is accompanied by accumulation of a passivating layer as a black deposit of complex hydrated Mo oxides or polymolybdates on the surface of via metallurgy. Such an oxide layer on the surface of conductive features is undesirable as it interferes with subsequent processing for example, deposition of follow-on joining metallurgy metal, thus impacting process viability for a functional electronic packaging component.
In the case of Mo etching at high pH(&gt;12) as with the potassium ferricyanide/KOH reagent, the alkali metal molybdate formed in the etch process remains soluble in strongly alkaline solution. With ferric sulfate or ferric nitrate, on the other hand, the reagent solution pH is maintained below 1 or preferably below 0.5 to prevent gelling of iron hydrates during Mo etching. At low pH, the initially formed molybdate (MoO.sub.4) undergoes in-situ self condensation to form complex oxides or polymolybdates which passivate the surface of molybdenum features such as metal vias resulting in a dark or black deposit on Mo metallurgical features.
With the above recited problems that are associated with Mo etching or removal using low pH etch reagents, such as ferric sulfate, ferric nitrate, and related materials, a need exists for a non-cyanide method to remove frozen or lodged in molybdenum debris from substrates such that the method does not suffer from the black via problem.
Bearing in mind the problems and deficiencies of the prior art, it is therefore a purpose of the present invention to provide a method for removing extraneous Mo from substrates using a non-cyanide containing etchant without damaging the Mo features on the substrate.
Another object of this invention is to provide an environmentally friendly method for removal of extraneous Mo from a ceramic substrate surface which does not leave a dark surface layer on the Mo metal features of the multilayer ceramic substrate which would interfere with follow-on joining metallurgy.
Another object of this invention is to provide molybdenum-containing electronic components such as semiconductors, metal masks, photomasks used to make electronic components, and the like using the method of this invention.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.